The definition of Yale’s blue color has varied over the years since it began to be used in the 1840s, but it was usually described as a dark shade of blue in sources from the late 1800s and early 1900s like the World Almanac. Yale’s shade of blue was also sometimes described as “Oxford blue”, which Oxford University defines as a very dark blue. A 1972 Intercollegiate Bureau of Academic Costume (IBAC) source described Yale blue as “dark royal blue”. That said, Yale tobacco cards, silks, flannels, pennants, postcards, and other memorabilia from the first quarter of the 20th century display a variety of medium to dark shades of the university’s blue.
Citations in the World Almanac (listed by cover date; color information is from the previous year): dark blue (1895-1901); blue (1902-1913); Yale blue (1914); Oxford blue (1915); Yale blue (1916); “Yale blue, known as Oxford blue” (1917-1918); blue (1923-1935)
In 1894, Princeton invited other universities to join an “Intercollegiate Commission on Academic Costume” to draft a uniform system of academic costume for American colleges and universities. Representatives from Columbia, New York University, Princeton, and Yale served on this committee, which used Columbia’s system of academic costume as the basis for the new “Intercollegiate Code of Academic Costume” the Commission approved on 16 May 1895.
Yale University formally adopted this Intercollegiate Code later that same month, stating that the lining of its hoods would be dark blue. In June 1901, Yale amended this regulation to say that its hood lining would be “dark blue of the shade known as the color of the University of Oxford.” Yale sent fabric samples to the Intercollegiate Bureau of Academic Costume (IBAC) and Cotrell & Leonard (the academic costume firm where the Bureau was located), who simply described it as “blue” (the IBAC described the dark blue Faculty color for Philosophy the same way). By the middle of the 1920s, the IBAC was using “Yale blue” to describe the dark shade of blue it was assigning other colleges and universities, even though Yale’s dark blue hood lining was still described as “blue” in the same Intercollegiate Bureau lists.